[TC6] Citroën Activa

[5imon]

This will (hopefully) be my entry for the TC6 contest. Seeing all the great models already under construction (or almost finished), I don't see a lot of chances as a newbie, and I was thinking of withdrawing. So I thought it's a good idea to start a topic here and get me some peer pressure to finish ;-)

As a big Citorën lover, the idea to build one of my favourite cars for this contest came quickly. But which vehicle would incorporate 4 independent PF functions? My choice was the Activa concept car from 1988:

Shown at the 1988 Paris Motor Show, it showcased the electronically steered hydropneumatic suspension that was to show up the following year in the Citroën XM. Furthermore, it featured active anti-roll, four wheel drive and drive-by-wire four wheel steering with different modes, including crab steering. More info on http://www.citroenet...iva/activa.html

The steering and suspension being the most interesting part for a Lego model, I chose the following 4 functions:

_ driving (L-motor)

_ front steering (servo)

_ rear steering (servo)

_ ride height (2 servos).

I already finished the prototype of an axle (the two axles, having the same functionality, will be very similar; only difference will be that the front axle will include also the driving motor.

Ride heights:

The height adjustment mechanism was a bit of a challenge, as it has to move quite some weight on a small lever. It took me some time to brace the whole mechanism correctly. The next big challenge will now be to put the two axles in a strong enough chassis. The most difficult part however are the smooth, rounded lines of the Activa's body. I'm more at ease with mechanisms than with shapes, so let's see how I'll be doing...

Comments and tips appreciated!

Edited January 3, 2015 by 5imon

[LeocornoProductions]

Good start

[Lipko]

Very nice concept for the contest. I had the same concept, but for a rock-crawler.

[Epic Technic]

Cool car, its gonna be hard to replicate those curves. Why the fake servo ?

[5imon]

Yeah the servo dummy, forgot to write about this. The additional servos I have ordered are still on their way... So I had to construct a placeholder for my prototypes.

[Doc_Brown]

That is looking really nice. Great job so far.

[Rishab N]

Never seen this car before and looking forward to more progress

Edited January 3, 2015 by Rishab N

[sm 01]

That's a lot of servo's.

I like your adjustable riding height function! This is a very original model (great idea) and I'm looking forward to see more.

[PKW]

This car should be fantastic with the proportional control (with an sbrick) of the high adjusting servos *-*

[TheItalianBrick]

using a servo for the adjustable riding height means to drive the car holding one of the remote control stick (not the adjustable ones) in order to keep one of the 3 position....right? so why not using an M motor so it can have different heights?

Edited January 10, 2015 by TheItalianBrick

[Epic Technic]

using a servo for the adjustable riding height means to drive the car holding one of the remote control stick (not the adjustable ones) in order to keep one of the 3 position....right? so why not using an M motor so it can have different heights?

Well this could be the SBrick improvement for it!

[TheItalianBrick]

Well this could be the SBrick improvement for it!

I know but I was thinking that somehow to use the normal remote controls it's a bonus as most of people has these ones...not the adjustable ones...so what about if someone wants to build this car after the contest? Lol

[5imon]

I considered different options for driving the ride height. It had to be a servo because I wanted the system to have absolute height positions. This is difficult to achieve with M or other motors.

I have now opted for an adjustable remote control for the height (and driving as well), so the height can be adjusted in finer steps and stays without holding the control all the time. In the end, the car is designed clearly with the use of sBrick in mind, and with this it's also possible to steer it in that way.

Btw, I have almost finished a running chassis, but I have to implement some modifications against heavy gear grinding at the differentials. More pics soon...

[Jim]

Nice work so far. Hopefully you will be able to enter the contest. I know there are some great entries, but that shouldn't stop you!

And there's always the extra SBrick which can be won by anyone

[5imon]

Thanks for the encouraging words Jim! Actually, I'm not very afraid of entering the contest any more. As I have completed most of the technical part, it turned out that it works quite well and looks good, too. So my only sorrow now is that the body might become too ugly or too heavy...

And in the end it's really about participating more than about winning. I've never built such a big model in a short time, so this alone is a completely new experience for me and lets me learn a lot. Plus following all the other entries is also very inspirational and instructive.

[5imon]

After a little research, I found quite quickly the reason for the gear grinding in my drivetrain.

This is how the transmission to the differentials was set up:

The longitudinal driveshaft crosses under the differential and drives this via a 12T double bevel (on the driveshaft) and a 20T double bevel (just above, in front of the differential. The differential itself is barely visible behind this 20T. The problem was that the two gears are held on the two black #2 connectors that are not directly connected. Furthermore, the top #2 connector with the 20T can also easily be twisted on the two angled dark grey connectors on the side. But the worst thing was that those 2 connectors could easily be pulled out to the top, leading to this situation:

So I created a new solution where the two gears are braced much tighter:

I haven't implemented this yet, but I strongly hope this will increase the torque that can be transmitted.

I guess that the weak point stays the axle pin that holds the 20T gear. It still allows an amount of flexing. I could use a 3L axle for the gear, but the end of the axle could not be fixed – next to the red connector on the picture is a pinhole of the servo motor which could guide the axle, but is closed at the back, so I can't fix the axle at the other end. I would have a "floating" axle with the gear that would only be held in position by the bevel gear of the differential. Maybe one of the experienced builders here have an idea which solution is better... Ideally, I'd have a 3L axle pin like the dark grey one that came out recently, but without friction. I guess with the friction, it would eat too much of the motor's torque.

Edited January 11, 2015 by 5imon

[PKW]

http://i1265.photobu...951336D4D9F.jpg this soluti in Could be better? Only because I think that To increase the supported torque you need To fix the yellow gear covering it also From the back, spining the differental it could tilt himself and lose grip in the down gear, but I'm talking of a really high torque. Edited January 11, 2015 by PKW

[Rockbrick]

Nice height adjustment but I think no one has come as close to the citroen hyrdraulic suspension height-correction but Sheepo's Peterbilt Truck:

http://www.sheepo.es/2012/11/peterbilt-379-cat-c15.html

[LeocornoProductions]

Anymore pictures,of progress because this seems like an interesting moc

[5imon]

Nice height adjustment but I think no one has come as close to the citroen hyrdraulic suspension height-correction but Sheepo's Peterbilt Truck:[/url]

I really have to look closer at this truck when I have some time. I always dream of coming closer to the function of hydropneumatic with Lego. Thanks for the tip Rockbrick.

Anymore pictures,of progress because this seems like an interesting moc

Thanks for the compliment Leocorno. But you have to be a bit patient for a few more days. I will get my new servos Wednesday, so afterwards I can finish the chassis. Then it's also time for the next photos.

[Rockbrick]

we look forward to more photos indeed!

[5imon]

Servos are in my car! Can't wait for finishing work and heading home for a building session!

[5imon]

Good news: the chassis is completed and works – almost! I still have to implement some reinforcements around the central differential so it can run without gear grinding. But everything else worhs smoothly :-)

I hope I can at least shoot a few pictures, unfortunately it's always dark when I'm at home this time of the year.

So stay tuned for thonight...

[5imon]

Finally! The chassis is up and running. For better bracing, I just had to add two liftarms, but that meant to dis- and reassemble almost the whole rear axle. I'm getting really proficient on that ;-)

It really runs well now, all functions work as intended, and with the train remote for driving and height adjustment, it's a really smooth driving experience. I could even gear it up a little so it reaches a quite decent speed. Let's see if this is still the case with the whole body on it, maybe I'll have to change some gears again.

But now, the pictures – sorry for the really low quality, I was quite in a rush and it was way too late for daylight.

View from the top – You see that the connection between the axles is still very preliminary. Though I was still astonished about how little the frame bends. Of course the position of the IR receivers is not definitive yet – but all the other components (motors, battery box) are. I expect very little changes on the chassis from now on, probably just the addition of a few attachment points for the body and interior. I plan to add stiffness mainly by a very well braced central tunnel.

Side view:

A detail from the back:

... and from the front. Here it becomes also obvious that it's very easy to chenge the main gearing of the drive motor. I plan to add an easily detachable front end to ensure this possibility.

And finally, some might also be interested in a bottom view:

Edited January 16, 2015 by 5imon

[MajklSpajkl]

Are you sure about the gears on the drive motors not being braced? I can hear some grinding of the gears in your near future ;-)

Edited January 16, 2015 by MajklSpajkl